These conductivity measurement devices have been known for a long time from the prior art in different circuit topologies and are used for the most varied applications. Here for example the industrial process engineering is important in which the conductivity of the fluids is checked in order to ascertain the concentration of dissolved salts in chromatography processes or the like. Typical specific conductivities in the range between roughly 50 mS/cm and 200 mS/cm can be detected here. In the field of control and monitoring of ultrafiltration processes conversely for example a range between 5 μS/cm and 100 μS/cm is especially relevant.
Another important application is the cleaning of process installations (for example the monitoring of flushing and cleaning processes), these installations being typically flushed with high-purity deionized water and the conductivity of this flushing water then being an indicator of reaching a desired cleaning state. The conductivities to be detected here are much smaller, typically less than 10 μmS/cm, certain requirements necessitating measurement errors or tolerances less than 0.5 μmS/cm. If for example, in this cleaning context, not only the flushing water, but also in addition for example a cleaning solution are to be monitored (typically somewhat heated NaOH, with conductivities of more than 100 mS/cm), in the conductivity measurement devices which are suitable for this purpose there is for example a need for systems which encompass a wide range of values of the conductivities of the relevant liquids to be detected.
The prior art furthermore discloses different measurement principles for measuring the conductivity in aqueous media. Thus for example to detect higher conductivity ranges inductive measurement methods are conventional, in which there is no (galvanic) contact of measurement electronics to the medium to be tested. But in particular in applications of the aforementioned type in which for example high-purity water is to be monitored in the microsiemens range, contact measurement methods are necessary, typically implemented by the feed and subsequent measurement of a measurement current via electrodes into the medium.
Due to the low conductivity of the medium, even at high electrode voltages the effective measurement currents are low, and moreover it can be assumed to be known that the current to be fed is provided as an alternating signal in order to keep low a voltage drop on the electrodes on the interface to the aqueous medium (relative to the actual voltage drop in the medium); electrolytic degrading influences on the electrodes are also avoided by this alternating signal.
It is considered known from the prior art to use in the so-called 2-pole measurement the same electrodes (as an electrode pair) both for the feed of the current and also for the measurement, and for example at low conductivities and accordingly optimized electrode geometry (typically then large-area electrodes with a small spacing are used) moderate injected voltages will yield easily measurable currents.
Then the specific conductivity (sigma) can be determined as:sigma=G×k=k×I/U, the cell constant k in the dimension length−1 (typically 1/cm) indicating the special conditions of the respective measurement cell configuration, included electrode parameters and electrode geometries.
When the measurement range is expanded to higher conductivities, for example several 100 mS/cm, the voltage drop increases on the boundary layers of the electrodes, at the same time the voltage drop decreases over the aqueous medium which is to be measured. To solve these problems it can also be assumed to be known from the prior art to decouple the electrodes for a supplied current from the measurement electrodes for measuring the resulting voltage drop in the medium, in addition to increasing the measurement frequency (therefore for example a polarity reversal rate of the injected alternating signal). This leads to the fundamentally known approach of the 4-pole arrangement, by analogy to the generic 4-pole resistance measurement technology, a measurement current as an excitation current being fed via a first electrode pair into the aqueous medium (electrolyte) and then the measurement voltage (voltage drop) being detected via a second measurement electrode pair which interacts simply via the liquid medium. Both electrode pairs are thus three-dimensionally spaced apart from one another and separated. Since on the measurement and evaluation side there is typically a high internal resistance of the downstream detector electrodes, small currents flow accordingly in the output-side measurement circuit, with the advantageous action that the voltage drops on the interfaces of the measurement electrode pair are negligible. Since on the primary side (i.e. in the circuit which interconnects the injection electrodes) a current which flows there can be measured and managed, the primary-side electrode voltage drop is not important.
While, as described above, the 4-pole technology is favorably suited for measuring the electrical conductivity in aqueous media especially also for high conductivity values, it is nevertheless conventionally complex to implement this technology such that it encompasses a large conductivity range. In the practical implementation it has been found to be problematic that the cell constant (k) which is determined and calibrated for example generically using standardized environments is dependent on the actual conductivity. In addition the interfaces of the electrodes, which for example depending on the respective order of magnitude of the conductivity, cause current density shifts on the electrodes and in the electrolyte, as well as alternating current effects, play a part, such as for example parasitic capacitances (as loading of the secondary side voltage electrodes) or crosstalk, therefore an unintentional alternating current coupling between the primary side excitation circuit and secondary-side measurement circuit. Here for example in practice the (often long) feed lines between the electrode and measuring amplifier significantly influence these parasitic effects.
These problems are solved in the technologies which can be assumed from the prior art by evaluation-side measuring amplifiers which are specifically matched to different measurement and value ranges of the conductivities to be detected and are suitably switched over (manually or electronically); typical quantities to be influenced are respective frequencies, trigger currents and gains of a measuring amplifier. Integrated designs, for example microcontroller units, make available comfortable infrastructures which make a multistage adaptability to various conductivity ranges easily programmable.
One problem associated with this technological approach is in any case the necessary occurrence of discontinuities or jumps at transition sites of these measurement ranges; if therefore there is an aqueous fluid to be measured in this transition range, potentially unusable measurement results arise. Therefore the implementation of hysteresis during switchover is necessary, however such (also can be favorably implemented with digital electronics) a hysteresis approach entails the problem that a value which lies in the hysteresis range is dependent on the direction from which the measured value changes so that a hysteresis interval or deviation means a potential measurement error.
Just the aforementioned digital electronics in which for example digital processes electronics also generates a (rectangular) primary-side trigger signal for injection into the aqueous medium entails other problems of practical importance. Thus for example the harmonics of the base frequency which are associated with a rectangular pulse signal are especially greatly influenced by nonohmic interference, for example the aforementioned parasitic capacitances. This in turn can be opposed only by a sinusoidal (at least low-harmonic) primary-side triggering; this distinctly increases the hardware cost.
Finally, the possible variation of a measurement frequency during switchover of the range which can be assumed to be known (for example within the framework of an integrated-digital design) entails the potential problem of ambiguity by parasitic capacitive effects: If for example at very low conductivities a very high measurement frequency is used, these parasitic capacitive influences can feign high conductivity, with the unwanted effect that the secondary-side measuring amplifier does not recognize the appropriate measurement conditions and accordingly does not undertake a pertinent measurement range selection or switchover.